A live ball is any ball that is active in the field of play. The ball is considered live from the moment the center snaps the ball to the point where an official signals the play dead. A live ball is fair game for any player to make a play on and can be advanced by either team. Play continues until a whistle is blown and the ball is marked "dead".
A live ball means the ball is in play, and action can commence. When the ball is live, yardage can be gained by whichever team is in possession of the ball. That team can advance the ball until the point where the referee signals the play is over. A defense will try to gain possession of the ball while it is live, while the offensive player in possession of the ball will endeavor to gain as much yardage as possible without losing the football.